1. Field of the Invention
This invention relates generally to an apparatus for accurately aligning first and second elements. More specifically, it relates to an apparatus for accurately aligning such elements without contacting the surface of the elements being aligned.
2. Description of the Prior Art
In the manufacture of integrated circuits, after a semiconductor wafer has been lapped and polished and a coating of photoresist placed upon the wafer a pattern is exposed on the photoresist by projecting light through a mask, the mask having the necessary indicia thereon. As devices become reduced in physical size and as their number has increased in the individual chips on the wafer, alignment of the wafer to the mask becomes more and more critical, for as the density of devices increase it becomes necessary to increase line and edge definition and to eliminate as much as possible areas caused by misfocusing.
Wafer to mask alignment devices are well known to the prior art. For example, U.S. Pat. No. 3,645,622 issued Feb. 29, 1972, and U.S. Pat. No. 3,711,081 issued Jan. 16, 1973, both of which are assigned to the same assignee as the present invention, describes apparatus and methods for aligning a semiconductor wafer to a photomask in which a photomask is mounted in a fixed superimposed overlapping position relative to a gimbal supporting a semiconductor wafer. The gimbal is arranged such that it can be raised up to force the wafer against the mask to align the plane of the mask and the wafer. After this forcing together of the mask and the wafer, the wafer is removed a fixed distance below the mask for photoalignment.
An article appearing in the IBM Technical Disclosure Bulletin Vol. 14, No. 5, October 1971 on page 1604 and 1605 describes still another technique for wafer adjustment. In this apparatus there is disclosed a contactless, plane parallel adjustment of wafers to a mask in which an adjusting plate is interposed between the mask and the wafer so that the wafer can be moved by forcing an air jet against the surface of the wafer until the gimbal upon which the wafer rests is adjusted. Once the wafer is set by the air jet with respect to the adjusting plate, the gimbal is locked, the adjusting plate is removed and the entire apparatus moved towards the rigidly held mask until it is in a preferred position and a selected distance from the rigidly held mask.
Each of these disclosed inventions, though obviously providing significant advances over the prior art, have drawbacks. For example, in the described patents which require direct contact with the face of the wafer which is coated with a photoresist layer, the apparatus used to adjust and position the wafer, will also become coated with photoresist. Because such accumulations of photoresist, on the apparatus, is uncontrolled they will eventually cause the wafer to be positioned divergent from the mask and the desired parallelism of the wafer with the mask is not realized. Moreover, if particles are trapped between the wafer and the holder they can when the wafer is pushed against the mask, seriously damage either the mask or the wafer.
In the apparatus described in the described Technical Disclosure Bulletin there are potential errors possible because of the complex mechanical movements required. Moreover, in neither case are means provided for continuously maintaining parallelism between the wafer and the mask.